1. Field of the Invention
The present invention relates generally to interference cancelers, and more particularly to a generalized sidelobe canceler, or adaptive beamformer for an array of sensors such as microphones and the like.
2. Description of the Related Art
It is known that wideband signals propagating across an array of sensors in directions that are different than the beam steering direction of the array suffer a distortion that is similar to lowpass filtering.
According to a prior art microphone array, signals detected by an array of microphones are lowpass filtered and summed together to detect a target signal that arrives in a particular direction. The adaptive microphone array beamformer is one form of the generalized sidelobe canceler as described in an article "An alternative Approach to Linearly Constrained Adaptive Beamforming", Lloyd J. Griffiths and Charles W. Jim, the IEEE Transactions on Antenna and Propagation, Vol. AP-30, No. 1, January 1982, pages 27-34. As described in an article "The Broad-Band Wiener Solution for Griffiths-Jim Beamformers", S. Nordholm, I. Claesson and P. Eriksson, the IEEE Transactions on signal Processing, Vol. 40, No. 2, February 1992, pages 474-478 (hereinafter referred to as Document 1), the generalized sidelobe canceler comprises, a spatial lowpass filter connected to an array of microphones for filtering signals from the array and summing the filtered signals so that only the desired signal is contained in the summed signal. A plurality of spatial highpass filters are provided to form a spatial highpass filter bank. Each spatial highpass filer is connected to a selected pair of microphones for filtering and summing the sensor signals to detect the interference signals. A plurality of adaptive filters are provided for using the interference signals as reference signals to detect those components having high correlation with the interference signals contained in the detected target signal.
Since the spatial highpass filters of Document 1 are of nonadaptive type and each uses two microphone outputs, the range of signals which must be rejected is very narrow. As a result, a slight departure from the intended direction causes a leakage of the desired signal into the interference path of the beamformer.
To overcome the prior art shortcoming, a proposal has been made to implement a spatial highpass filter for receiving more than two microphone outputs as described in an article "A Spatial Filtering Approach to Robust Adaptive Beaming", I. Claesson et al, the IEEE Transactions on Antennas and Propagation, Vol. 40, No. 9, September 1992, pages 1093 to 1096 (hereinafter referred to as Document 2). According to Document 2, each of the highpass filters that comprise the spatial highpass filter broadens the range of arrival angles by receiving multiple spatial samples from a selected set of microphone outputs using a plurality of leaky adaptive filters.
However, a large number of microphones (the Q value) are required to implement a beamformer having a wide range of rejection angles, for each group of spatial highpass filters in the filter bank. If a sufficient number of microphones is not provided, the degree of design freedom must be sacrificed, resulting in a beamformer having a low noise canceling capability. The difference between the assumed direction and the actual arrival direction of the target signal, or a look-direction error, is of another concern because it degrades the target signal, or a look-direction error, is of another concern because it degrades the target signal. In order to compensate for this shortcoming, the spatial highpass filter bank of the prior art needs as many spatial highpass filters as is necessary to provide a wide range of angles to reject the target signal to prevent its leakage into the interference path of the beamformer.